feat(mobile): implement off-canvas navigation drawer and layout fixes (GIA-51)

[GianRomani]

Summary

Implements an accessible, high-performance off-canvas mobile navigation drawer and resolves the in-flow sidebar layout issues on viewports < 992px.

Linear Issue

Closes GIA-51

Changes

• Off-Canvas Drawer: Replaces bottom-dumped in-flow sidebar with a slide-over drawer triggered by the header hamburger button.
• Glassmorphic Backdrop & Scroll Lock: Adds blurred backdrop and locks body scroll when drawer is open.
• Accessibility & Micro-interactions: Adds aria-expanded, ESC key dismissal, click-outside handling, and auto-dismiss on link navigation.
• Mobile Content Layout: Resets .docs-content to 100% full width with proper padding on mobile devices.

Local Decision Documentation

• .notes/decisions/2026-08-17-gia-51-mobile-navigation-drawer.md

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